There is a host of evidence showing CNS damage from drug use/abuse (DA) and HIV/AIDS. However, the majority of these studies assume that it is the direct effects of these factors on the CNS that result in altered cognition. The purpose of this exploratory study is to evaluate a different mechanism - namely, that alterations in vascular function are at least partial mediators of the effects of DA and HIV/AIDS on cognition. If the overall hypothesis is correct, that means that many of the CNS changes associated with DA and HIV/AIDS are secondary to vascular changes, and this might explain why there are synergistic effects between these conditions on cognition, and would suggest pathways for the treatment and prevention of cognitive changes in DA and HIV/AIDS that have not been typically exploited. In order to evaluate the hypothesis that alterations in CNS vasculature are at least partially responsible for the cognitive changes in DA and HIV/AIDS, we will take advantage of a novel technique available through magnetic resonance imaging - Arterial Spin Labelling. This technique allows for the measurement and mapping of regional cerebral blood flow (rCBF) without the use of contrast agents or ionizing radiation. In addition, by challenging the subject to breathe CO2 during the scan, it is possible to measure the acute vascular response to hypercapnia. The purpose of this exploratory study is to allow us to obtain the data necessary to develop a larger scale, multi-center study of the association between vascular integrity, DA, HIV/AIDS and CNS structural and functional integrity. Before using these state-of-the-art methods for measuring both "acute" (i.e., hypercapnic) and "chronic" (i.e., resting) rCBF as biomarkers for tracking disease and for investigating the underlying pathophysiology of HAD and associated conditions, it is necessary to establish some basic information regarding the stability and validity of the techniques. The purpose of this study is to evaluate rCBF in individuals with HIV/AIDS with and without concomitant drug abuse. Specifically, we will determine the extent to which rCBF is stable over 24 weeks of study, and determine the criterion validity of the MRI technique using concomitant O15 -water positron emission tomography (PET) measures of RCBF. Individuals with HIV/AIDS may have difficulty thinking. Individuals who use drugs may also have difficulty with their thinking. It is also true that people with HIV/AIDS or who use drugs may have problems with their circulation (blood vessels). This could mean that not enough blood is getting to the brain, or that the blood vessels in the brain do not respond correctly when the brain needs more blood. It is possible to measure blood flow in the brain using a technique called magnetic resonance imaging, which can take pictures showing how fast blood is moving through various parts of the brain. The purpose of this exploratory study is to begin to use this new technology to study blood flow in the brains of people with HIV/AIDS and drug abuse. We are interested in knowing whether changes in their circulation may be responsible for some of the changes in their ability to think.